Examples from...

Religion comes up in one chapter, and Tony explains he doesn't go to church because of an incident involving a coffee maker. Apparently they still haven't gotten the stains out of that church's ceiling, though Tony's father was very proud he managed to get the thing airborne.

Also from that same fanfic: Tony can't get any pets until every last gerbil has come out of orbit. Tony's response: "So if the squirrel doesn't come back, that's okay then?"

In the Fourteen Days fanfic, everyone comments on how much of a pervert Charlie is by stating a few occurrences along with the phrase: "and don't forget the pasta incident"

The circumstances behind Francis getting arrested in Comedy of Errors was initially this. One of the one-shots in Hearth explained it after the trilogy was over, though. Francis proposed a threesome with Tino and Berwald, and Berwald threw things at him. When Vash arrived, he saw the mess and arrested Francis for it.

In a footnote, it was mentioned that Vash only had to fire his gun once, since he was made the sole policeman of Hearth. "At Ivan. Right before Alfred, Arthur, and Gilbert restrained the Russian for him. To be fair, Ivan was just trying to protect himself. Natalia is scary."

There is only one bench in all of Hearth. According to the author, the readers don't want to now what happened to the other ones.

When explaining why Ludovicus has no accent when both of his grandsons do, the author mentions that he had to learn to speak English perfectly to keep Marcus from speaking on his behalf, lest the two of them get arrested. Again.

The exact circumstances behind Kiku and Vash's homes adopting the personalities of Ludovicus and Marcus respectively. All we're told is that each of the men had been visiting the town, and the AIs programmed into the houses decided to model themselves after them.

There are several more serious ones referenced in regards to Gilbert and Ludwig's backstories. We know that Ludovicus took his grandsons from his father's care, but it's never explained why. It's also implied that Ludwig had some sort of accident that resulted in partial memory loss, but again no details are given (it's likely a reference to the fan theory that the Holy Roman Empire was Germany's younger self and lost his memories, explaining why Germany couldn't remember Italy). There's also an incident mentioned when a five-year-old Feliciano visited the two at their farm, which goes unexplained.

Spider-Man:(about Green Goblin) Honestly, who's stupid enough to get that drunk? Iron Man: Look, Comic-Con went really well, all right? I've been very good, everybody was celebrating, everyone was doing it, we thought we could make it to Tijuana, they dropped the charges, so just shut up, okay?! (awkward silence)

In Pony/Pokemon crossover A New World A New Way - Swarm, Azisa is forbidden from sleeping in the treehouse of her trainer due to something referred to only as "The Volcarona Incident"

In Shinji And Warhammer 40 K, Ayanami Rei, Kirishima Mana and Ibuki Maya all decide to relieve Shinji's pent up teenage stress in order to up his sync ratio. The following events demolish several buildings, and both psychic warriors and terminators had to be deployed.

In Jedi Harris, an employee at Wolfram & Hart asks about bullet holes outside one of the labs, to which a technician mentions an accident opening a hole in time and space. The employee's sole response is, "Oh hell, not velociraptors again?"

In the second chapter of Weiss Reacts: Volume 2, Kyubey makes reference to his disastrous visit to Inaba. Despite the awesomeness such a meeting would clearly result in, it is never mentioned again.

In A Charmed Life one of the demands the Japanese police make of L is that Matsuda gets to keep his emergency belt so he doesn't get locked in the men's room again. Also Ryuk and Light make frequent stops back to the human world to buy groceries and home furnishings.

In Death Note II The Hidden Note, Sayu starts to talk about how she, Catherine and David ended up living together. As she does, we hear Ryuk talking to KJ about finding an apple. By the time that conversation is done, Sayu is finished explaining the story.

In Fever Dreams Matsuda has apparently previously gotten in trouble for giving out his e-mail address to suspicious characters such as a suspect in an ongoing murder investigation:

Light: I could send you an email.

Matsuda: No, we're not supposed to do that. Then everyone would get mad at me and I'd have to change emails again.

Light was actually quite proud of himself—he hadn't punched a hole in the screen yet. Unlike that one incident when he was around six years old and needed to get stitches afterwards and no one ever talked about and never happened.

And so began the Great Butterbeer Incident of '96. Many of those in attendance refused to speak of it ever again, those who will respond to inquiries will reply with only one descriptive word...

Everywhere.

In Harry Riddle Harry and Draco shared a traumatic childhood experience of getting locked in a Gringott's vault for hours.

In Invictus Harry relates that "centaurs tend to be grumpy in the mornings, and no, please don't ask me how I know, as it's a long and humiliating story involving whiskey and multicolored shrubberies."

In Let's Do The Time Warp Again, there is mention of an incident where James and Sirius gave Snape an all-expense paid canoeing trip to Georgia and then stole his wand. Exactly what happened during the trip is never explained, but it caused Snape to lose both his arms and legs as well as develop a debilitating fear of banjo music. (If you're even slightly familiar with Deliverance, it should be possible to make some educated guesses about the details.)

In chapter 14 of Living Without Danger, the adults reference "The Great Hogwarts Noodle Incident," though they never explain exactly what happened (to the reader, at least).

"You mean you don't remember the Great Hogwarts Noodle Incident?"

Letha's eyebrows rose. "The Noodle Incident -– that was you?"

Padfoot was sliding down in his chair as if wishing someone would Vanish him.

"Do enlighten those of us who weren't there," urged Danger, smiling wickedly.

"Yeah, please?" echoed the cubs.

Moony laced his fingers together. "Well, it all started with a comic book, an action figure, and a stuffed tiger..."

In Magical Relations Fred and George told Harry the third-hand story of the Gouda Catastrophe of '77 which involved a large quantity of the cheese, several mice and the Great Hall. They also mentioned the time the Marauders covered McGonagall's desk with catnip.

Slytherin: Remember the incident with the silver dragon? And the group of Basilisks? Not to mention the time we were overrun with Ashwinders? Gryffindor: The dragon went on her merry way, and if it hadn't been for the Basilisks, you wouldn't have gained that pet of yours, and as for the Ashwinders—

“Well, there might have been an incident seventh year during a Hogsmeade weekend when she was out with her boyfriend involving a frog, a bottle of pink hair dye, and a trampoline,” Sirius admitted. “Do tell,” Harry encouraged. “First of all, I fully deny any and all involvement. My presence -– along with James, Remus, and he-who-is-not-worth-mentioning -– was completely coincidental. As was James’ camera. It all started when…”

The first Potter's Pentagon story references one in chapter two. Emma Weasley goes on one of her tirades when a girl insults her friend, and Ted brings up a past Hogwarts incident.

“Impressive,” said Ted when the storm had died down. “Those were some of the most creative threats I’ve ever heard. They even top that thing last year with Professor Zabini and the potato.”

Simple Terrible Truth mentioned a snowball fight during the Marauders' time at Hogwarts which resulted in " plenty of bruises, an igloo, one lost and later found first-year boy and three chocolate cheesecakes."

In Unexpected Changes Sirius treated Harry to stories of past pranks while helping him pack to leave his relatives':

Sirius: …I'm turning Harry pink! It was brilliant! And then afterwards, well lets just say there was a chicken and a tutu. Remus: It's true, your father was very proud of himself, though I'm sure it was purple Padfoot. Harry: Yes, but really, why Professor McGonagall? Sirius: That's what makes it brilliant!

Luna: The last time I let the two of you go someplace on your own, you ended up facing down a pair of zombie dragons and wrecking three blocks of the Las Vegas strip. Harry: Totally not our fault Lu. How were we supposed to know that an evil tenth century necromancer had guarded his tomb with a pair of undead dragons? Besides, it only took a day to fix the casinos.

Chapter 5: Next Stop: Third Base of Had we But World Enough Loki and Thor are engaged since birth, but after being celibate all this time, they learn they must conceive a child or Universal War will be waged by Loki's brother. Tony opts to help out; inadvertently leading to disaster and suggests a romantic carriage ride, leaving the two alone to enjoy it. The following rant from Fury pretty much explains it...

“A fountain destroyed, over 20 thousand dollars in destroyed plants, garden bed and walk ways, numerous people scared out of their minds, let’s not forget one traumatized carriage driver and horse not to mention the carriage which, as far as we know, is still wrapped around that tree and finally, one thirty million dollar avant guard art installation which no longer graces Central Park…”

In X-Men fanfic Mutatis Mutandis by Artemis's Liege, it's never told what happened between Manuel de la Rocha and Kitty Pryde, but his words and actions indicate that he deeply hates her.

While working for Fluttershy, Trixie tried to get eggs from one of the chickens, named Gertrude, who she was warned was temperamental. We don't see what happened, but apparently she tried to pry her off her eggs with a crowbar and ended up getting thoroughly beaten up. Trixie insists that Gertrude was a "ninja chicken" and tells the others to never speak of it again.

The Cutie Mark Crusaders get two when Trixie is working for Rarity. Sweetie Belle tells Twilight about the time they were "Cutie Mark Crusader Tennis Players", all we know is they got banned from the tennis court. Later, the other Cutie Mark Crusaders assume Sweetie Belle wrecked Rarity's shop again because of a previous incident when they tried being "Cutie Mark Crusader Basket Weavers" and them ending up in a wrecking ball. Apparently, it also involved something happening with a bird that got Fluttershy mad at them and they ended up covered in tree sap... again.

In Equestria: A History Revealed, when discussing the supposed orgy that brought down the Equinus Republic, the narrator states that a reason why such an event was kept quiet was most likely to prevent a resurgence of the orgy craze from ten years ago. While it sounds as though such an event would be Exactly What It Says on the Tin, the way it is so casually thrown in there makes one wonder what the backstory to that event must have been.

In Friends of a Solar Empire, there is mention of Fluttershy joining Rarity's crew in some rather over-the-top antics during the last shore leave. Just what they did is never mentioned.

Nightmare Moon refers to a "silence weed incident" which apparently should have pushed their mother to disown them. Of course, Celestia knows exactly what she's talking about from the mere mention.

Arcanus's recount of a past wedding in chapter 4 is loaded with these.

This story has Twilight apologizing for a "graduation party incident" that Shining Armor had reprimanded her about.

The fic My Little Parcel takes place in the aftermath of Twilight and friends' latest adventure, which involved pirates and sea monsters, and of which only tantalizing hints are given.

In the Pony POV Series, Applejack gives us a few details of a rather crazy adventure she had in Canterlot. It involved the ghost of a Sea Pony, a lost locket, and Apple Bloom being transformed into a Sea Pony for a short time. Not even Applejack is sure how that last part happened.

Princess Celestia, being a Time Abyss, mentions these, but seems to have wracked them up during her reign with a strange amount of frequency. Such as two separate assassins knocking on her door before entering, a pair of stallions disguising themselves as her to sneak into the royal treasury, and her personal assistants turning out to be a spy, an assassin, and going mad with power after realizing she could control what Celestia sees. And then there's 'the Century of Chubby Celly.'

Ditzy Don't is full of these, most of them having to do with how Ditzy somehow screws up every attempt to help the Mane Six (usually setting something on fire in the process).

When Spike and the Mane Six first meet Wavedancer, mute at the time, the following exchange occurs:

Rainbow Dash: What’s the matter? Cat got your tongue?

Rarity: You leave Opalescence out of this! She only did that once, and she was sorry!

Jericho has this in shades of dark to light. The narrator, Jericho, is a bit of an odd pony.

The idea of that reminded me of the first case I’d ever worked on, where a psychotic professor at some prestigious medical school had been going around murdering girls, stuffing their dead bodies full of extra organs because “aliens told him to do it.”

The Nuptialverse: There are two words explaining why alcohol was banned from Equestria for five hundred years, causing a decade-long economic depression: drunk Luna.

Hands: According to Andrew, the CMC once tried to build a spaceship. He doesn't elaborate, saying he doesn't think he can without descending into cursing.

Apparently, a month before the start of the story, Twilight, Pinkie, and Applejack went camping, and Pinkie ended up accidentally burning one of Twilight's books, causing her to have a Freak Out that outshined the one she had in "Lesson Zero". AJ and Pinkie calmed her down, but both swore they'd never go camping with her again.

Celestia and Luna both refer to earlier encounters with Kryptonians, with Luna saying that she owes them a "great debt".

Princess of Themyscira:

A more serious example, there's The War in Heaven and Athena's encounter with the forces of Apokolips.

Diana lists a few from hers and Cadence's childhood, such as when they accidentally set the armory on fire, and another time when Cadence intentionally set it on fire.

In Brightest Day, In Blackest Night:

Apparently, Cadence broke each of her legs at various points while being raised by the Amazons. But Diana always got it worse.

The Last Crusade an energy drink binge is referenced in dialogue, but the narrator feels too ashamed of whatever happened during it to give context.

“Think of it as a memento,” Dylan said with a wink. “Though I doubt any of us could forget that weekend even if we tried.”

What he was referring to was—you know what? Never mind. Trying to describe some things with words simply doesn’t do them justice. All you need to know is that we were each grounded for three months, no charges were filed, and we made the front page of the local paper.

"Dear Sweetie Belle" opens with Sweetie Belle coming to live with Rarity after their parents die in an accident, the circumstances of which are never explained. The author imagines it was a carriage crash.

It was nice how the mare wasn't shy towards her anymore, although the incident of how it occurred would never be brought up again. Just remembering the mess she had to clean up... no, she wouldn't let her brain bring back up that image. She did feel sorry for that squirrel though...

During the epilogue of You Can Fight Fate, Luna shows up excited about a planned adventure, but Azalea refuses to let Twilight get involved because she's pregnant.

Luna: Twilight! We have been fools this entire time! "Argent Map' is an anagram for the word 'pentagram!'"

In one of the early chapters, Spike mentions that the library's oven is out because it blew up the last time Twilight tried cooking. She insists it was a design flaw.

Rarity's off-screen exploits with her nemesis and business rival, which somehow ended up causing her to be deathly afraid of cheese. And the incident which caused said nemesis to die, then come back from the dead some time later.

At one point during a "Freaky Friday" Flip-situation Rarity offhandedly mentions a time Rainbow Dash tried her hoof at dressmaking. All involved swore never to mention it again, and Rarity doesn't divulge anything else.

Luna's one and only attempt at attending the Grand Galloping Gala. It apparently ended in molten cake.

Diaries of a Madman has a griffon mention the minotaurs being banned from an event due to a noodle incident.

Earth and Sky refers to a whole string of these in chapter 39. Apparently, Trixie didn't quite learn her lesson after the incidents of "Boast Busters" and "Magic Duel", and tried to out-do Rainbow Dash as "The Awesome Trixie" and tried to upstage Rarity as "the Fabulous Trixie". Trixie also butted heads with Fluttershy in a "magic rabbit incident" that Applebloom complains had them "cleainin' lettuce outta the rain gutters fer days."

Star Trek Fan Works

The summary of Atomicskull's Star Trek AU fic Citylights is this: "World War III (known as Almost the End of the World) occurred in 2014 during Milan Fashion Week. It was short, it was bloody, and it was absolutely spectacular to see unfold." We never do find out exactly what happened. The final part of the series suggests it might have had something to do with investing. Or taxes. The jury's still out.

Dul'krah mentions a prior break-in at Facility 4028 carried out by a rogue Starfleet group with a Section 31 obsession. According to the author's notes this was meant as a Shout-Out to the Star Trek Online Foundry mission series Star Trek: Allegiance.

Apparently Eleya's in hot water with the Federation brass right now because she cussed out Tuvok, three ambassadors, and Proconsul D'Tan (presumably during the conference in "Surface Tension"). Word of God on the Star Trek Online forums is that the Fix Fic for "Surface Tension" explaining this is in development.

Brokosh and Eleya apparently fought each other at Regulus IV when she commanded the George Hammond. Also references to the IKS QarchetvI', a Negh'Var-class battlecruiser taking part in the attack on the Terran base, leaving the USS Hamburg for dead and nearly blowing up the Hammond even after Eleya tried to surrender.

Eleya is confident that Commander Roxy will give her a unit of MACOs because Roxy owes her a favor. All T'Var will say about the matter is that they "extracted her from a predicament eighteen months ago." The rest is classified.

"From Bajor to the Black, Part II" has Eleya airily refer to having woken up next to two passed-out Klingons with a hangover, a cracked rib, and "several bruises in embarrassing places". The author commented that "tequila was involved".

Peace Forged in Fire has Played for Drama example. Sahuel t'Khnialmnae is apparently ex-Tal'Shiar, and was aboard the Aen'rhien when it killed hundreds of refugees in an unelaborated incident.

The incident was the subject of its own one-shot. To poor Demeter, however, whose mostly Fanon distaste for Tugger is legendary and who would have LOVED to see a large turkey decide Tugger was its mate it will forever be a Noodle Incident.

In Merlin fanfic A Very Hairy Situation, the several spurned lovers who attempted to kill Gwaine using nothing but a garlic clove and some grass (it's surprisingly painful).

First Impressions: Murdoc and his old cronies agreed never to mention "that thing you did with the cat and the watermelon and the box of fireworks" again. It never gets explained, but apparently it was at least marginally less disturbing than making out with a dead body (which he actually didn't do). May or may not be the same thing as "the thing with the cat" which he refers to later, which appears to have been somehow sexual in nature but was "taken entirely out of context" by the gossips.

In The Wasteland Life, a Fallout 3 fanfic starring a lone wanderer named Bren, he along with with a Former Brotherhood of Steel went into a metro in which Bren smelt something (a deathclaw eating a dead Super Mutant) that reminds him of "Butch's Science Project in the Eighth grade, it got an F minus and boy it was horrible to the Janitors who had to clean it up" creating a small noodle incident that makes you wonder what could have Butch possibly done at that point in the Vault.

"I am of course referring to some wonderful karaoke performances, flights of fancy while being high on cough syrup, a few fights at bars…" Cain went on. "And of course the most memorable of all, Captain Foxx's impersonation of a naked cowboy riding a mechanical bull…The rest of you nut jobs aren't exactly the most civilized drunks I've seen either we all know what happened the last time Ranger Bennett got drunk and what she did in the closet with four rangers, an ambassador, a pair of handcuffs and that toilet plunger! Please also observe the dress code. In other words stay dressed and no nudity! Also do not set anything on fire! I know it's a common sense rule but after last afternoon's little mishap in the cafeteria I think I should remind certain people that."

Referenced later, when Kuyou attempts to swallow a whole plate of noodles at once (and chokes), and Kyon refuses to elaborate when asked about it later.

In the Psych fanfic Carlton's Worst Inhibitions, Shawn says: "You'll notice that I did catch hoof and mouth in the fifth grade after a trip to a petting zoo. Interesting fact, llamas aren't as friendly as they appear."

Subverted in the Code Geass fanfic LivingContradictory with 'The Sword Incident', which is explained in detail right after it's first brought up. The trope is then played straight with 'The Beard Incident'.

In "The Ultimate Bet", there's "the incident with the rubber chicken, L, and banana pudding" and also the following:

Ide: Yeah but remember the incident in the office with: the fruit smoothie, the giant inflatable clown, the crayola pencils, the toilet paper, the 50 pounds of pixie stix, Matsuda, and the 15-year-old fangirl going 'MATSUDA-NII!'

Rule 19: We are not actually naming the incidents. If Special Agent McGee says that we are naming the incidents, he is wrong, and should be reminded of the Exploding Highlighter Incident. Shut up, McGee. Ziva, stop laughing. Abby, why are you here? Addendum to Rule 19: Only Special Agent DiNozzo can name the incidents. The official incidents include: the Exploding Highlighter Incident, the Ziva's Unexpected Soy Allergy Incident, the Don't Touch That Incident, the Curiously Zen Marine Sergeant Incident, and the Admittedly Heartwarming but Decidedly Unmentionable Flowchart Incident.

Pretty The World, a Bones fanfic, has one. In Chapter Four, the OCs discuss amongst themselves a trip to Tijuana that involved injuries, life scarring, hypothermia, barn animals, carrots and something with a pencil that didn't matter due to gel padding.

In the original, we're never told what, exactly, Prazza did in that issue of the porno magazine Fornax that got it labeled the "Forbidden Issue," but we are told that an elcor was involved, that it violated at least thirteen laws in Citadel space, that it turned one poor C-Sec officer off sex for good, that Prazza did it to try to raise money for a Pilgrimage gift and got disowned by his family because of it and was unable to complete his Pilgrimage as a result, that the issue in question caused many who viewed it to go crazy or commit suicide, that the owner of the magazine was arrested the day after the issue was published and the magazine got a tarnished image and a ton of lawsuits because of it, and that it was something that went against the law of the galaxy, if not the universe, and most definitely didn't do much to improve relations with quarians in general.

In the sequel, Project Gethinator, we learn from Nehallennia, the managing director of Fornax, that what happened in the Forbidden Issue was the brainchild of Fornax's first human director, whose work was massively popular before this, and that he conceived it as a kind of "magnum opus." On top of everything else that happened (Nehallennia mentions that even asari matriarchs were rendered comatose for weeks by the scene in question), it ruined the director's career and led to the suicides of three of the actors involved. Oh, and the name of Prazza's little scene in the issue? "Ride My Love Rocket."

Episode 10 of Futari Wa Pretty Cure Dragon, a crossover episode with Pretty Cure Mirai Spark, parodies this early on when the Magnificent Trio, who had just wound up in Millennium City, mention that a "pasta incident" at Western Paradise (though this being the Hong Kong Dub of the Curefic, this may be an inaccurate translation of the original Japanese, which simply mentioned Italian restaurants without being specific) was responsible. Said pasta incident is never elaborated on further.

From Adventure Time fanfic, Alex, the titular antagonist knows about the falling out between Marceline and Princess Bubblegum. That's because she instigated it.

In the Firefly fic Forward, the Six Rifles refer to an instance called the "Boros job" which involved a gunfight with twenty federal marshals, but the specifics are never given. Amusingly, this is during the "Last Man" story, which is actually exploring a canonical Noodle Incident, that being Jayne's story of "six men who came to kill me one time."

In Renegade, Zaeed is mentioned as destroying countless military bases, warships, aircraft, and "one very unlucky dirigible." On a more serious note, there's whatever happened on Akuze; the specifics are not stated, just that Shepard encountered the Scrin and the battle ended with her being so horribly scarred by Buzzers that the Scrin now terrify the hell out of her.

In YuGiOh! The Thousand Year Door Redux Shadow Spawn of Light Melissa once drank a Growht Potion and a Shrinking Potion at the same time to see what happens. It is made very clear that she doesn't like it when it is brought up.

In the Cat Returns fanfic The Lion's Warrior, Baron and Lune(as the crown princes here) mention the one time they ran away from the palace a few times though the story. The scene will begin to waver like a flashback is starting when they shout in fear while waving their arms at the reader/author "NO, IT'S TOO SCARY!!" One time, their dad the king(not the same as the movie) asked them what they were doing, to which they replied, "Nothing."

Some time between the end of the story and the Distant Finale there is a 'Summoning War' which Naruto remembers in passing when talking about how everything hasn't been exactly perfect since the big bad's defeat.

Some time before the the Nine-Tails attack the Leaf Village, the Demon was forced to attack and destroy the Village Hidden in the Lakes. The Fox regrets it because of all the weaklings she was forced to kill, but otherwise does not elaborate.

In Shadowchasers Ascension, it's never revealed what Ophelia's mother did that landed her in prison (and to this day makes Ophelia embarrassed to be associated with her) only that it was a very public spectacle aimed at drawing attention to herself, a habit that shadar-khar tend to have.

In Shadowchasers Backwater, Tsubasa has two: In Chapter 2 of Knightfall he and Neo discuss a deck he used in the past that focused on Shapesnatch, of all things. While he actually won the Shadow Duel where he used it, the criminal in question escaped as it left him too exhausted o pursue. We can only imagine how this strategy actually worked. Second, his entire family is no longer allowed in Las Vegas due to an incident involving "a blackjack table, some cheese, a ficus, and Hyde Vayne". All other details of this are classified in all known records.

Voltaire, the star of Shadowchasers Danse Macabre, is full of them. For example, was an incident with a giant squid, which he refuses to elaborate on, because his former apprentice threatened him with bodily harm if he ever told anyone about it (and to this day, she can't look at a plate of calamari without blushing, he claims).

In Shadowchasers Conspiracy, Edgar has a few of them, but there seems to be enough information to apply other Tropes to them:

Did You Just Romance Cthulhu?: He tells Maddie he “fought an erinyes temptress from Hell, outwitted another, and I think I might have dated another…”

Gladiator Games: He also mentions that he “fought in the Hall of the Vanquished both as a prisoner of the place and a second time because I was invited…” (Interestingly enough, this place is also mentioned by Zanth in Tournament of Shadows, but he doesn’t describe it either.)

Our Werebeasts Are Different: When he expresses his skepticism of The Greys, Maddie mentions how his mentor was a Shadowkind wildlife expert who “said the very idea of a lycanthrope who turned into an animal that wasn’t at least partially carnivorous was absurd”, and then mentions an encounter with a were-gorilla in Niger. (Edgar admits it, but then adds that he was “under a rather odd ancestral curse."

Speaking of odd were-beasts, in Shadowchasers Tournament Of Shadows Sonya recounts to Nicole an incident where encountered a "very confused wererat" who claimed he was a were-Tasmanian devil (marsupial lycanthropes don't exist) and another incident where she "met" Terra Ironhoof, but won't give the details.

Horseshoes and Hand Grenades has a sidestory called SplitxEnd that states that JK's currently reeling from "that incident" which caused the rift between him and his father.

Since the main character of This is the Life: A Tale of a Human in Equestria is meant to be an Audience Surrogate, any reference to his past will be something like "You sat back and filled them in on why you decided to move here and live with ponies". Also, at one point for some reason or another, he apparently tried to eat hay:

Even stealing the Eiffel Tower wasn't terribly impressive; it was large, yes, but all large things could be broken into pieces, and with enough people it could even be done quickly enough that nobody would be caught in the act. Interpol had already worked out most of the logistics of it, and though Nancy had to admit that using a submarine was particularly brilliant...

The Supernatural fic "The Heart and Stomach" includes the narrator thinking about her sister's planning face, which "had never failed to get me in trouble since the age of ten and that thing with the cornflakes".

Yang seems to have quite a few of these in React Watch Believe Yikes, probably due to her brash and aggressive attitude toward combat or close proximity to Ruby.

As The Prankster, Mike in Pokeumans triggers a lot of these: his 'soap bubble fiasco' is Asula's office, his first attempt at a Groudon transformation and numerous incidents involving 'Electrode bombs'.

In the Medaka Box fanfic World As Myth, main characters Zenkichi and Kumagawa got up to "something" ten years ago. It involved rice pudding.

A knight mentions an incident involving bedbugs where he had just happened to be in the same room and "had nuthin' to do with them". This never gets elaborated.

The Battle of Al'Zahur is mentioned on occasion but very little is known about it. The people who do know about it aren't about to talk.

At some point several expeditions are name dropped at Valkron, who apparently gained his reputation from them. He shuts the conversation down very quickly and they go unexplained.

The Homestuck fic If It Means A Lot To You never fully explains why Eridan went on a murderous rampage through his school. All we know is that he blames Feferi and Sollux for it, and broke off his friendship with the former several days before the shooting.

The Doctor Who fic Time and Space, written by Eureka2000, has four so far:

In Chapter 1, there's the "Wal-Mart Incident". Nothing is revealed except for the fact that it involved OC!Zanna and her brother, Garrett, and that it happened at Wal-Mart. Zanna also compares it to their encounter with a Weeping Angel, so it's probably just as bad.

In Chapter 4, OC!Epsilon jokes to Cass that she hopes they're not in another labyrinth while wandering the corridors in the Rani's TARDIS, and Cass replies by saying that she still doesn't know how the Rani managed it.

In Chapter 9, the Doctor and his companions enter the console room soaking wet and vowing to never speak about what happened. The only clues given about the situation was that they were not on the planet they were looking for and that the locals were dreadful.

In Chapter 10, Epsilon mentions some incidents involving Catherine the Great and the Space Mafia, which prompted her to choose a codeword to use when she thinks that the Doctor is about to lose his temper.

In the ongoing BleachFan FicPlease Stop Eating The Hell Butterflies, there is an uncountable number of these, such as whatever resulted in Aizen hailing Orihime Inoue as the "Great and Powerful Queen of Hueco Mundo" and cowering at the sight of her. There was also another incident involving Mayuri and some exploding babies. At one point, it was even invoked during the Halloween Episode, where the sheer chaos of the event made Yamamoto decree that no one was to ever speak of it again.

In one ''Kill la Kill alternative universe fancomic, we have what happened last Tuesday when Nui had a soda [2]. According to the flashback panel, whatever she did caused total chaos and destruction.

Also from said author and another full length comic, we get what happened to Nonon's monkey being a Dark Secret. [3] According to the description, they were fooling around and said monkey was caught in the crossfire and the fact it was an accident.

A recent one is where apparently Nui and Houka's minds went to self-destruct, requiring an ambulance to be summoned. From some indication, it's happened more than once.

Ragyo: Again?

Satsuki: Yes, again.

According to chapter 8 of Room 002108, there was an incident with a grenade that, in parenthesis, Nui insists she had nothing to do with, along with an incident about fireworks that Ryuuko was noted to sleep through.

According to Nui, the reason they couldn't stay with friends is because of something that occurred that "one time", remarking, "She's like how old?! Too damn old to be wearing that tacky dress, that's for sure."

From Aikurou Babysits, we have Senketsu's existence. According to Ryuuko, he crawled out of her dad's lab, although exactly how he came to be is unsaid.

In chapter 20 Gensokyo 20XXI, Yuuka mentioned an incident with Eirin involving her, Tewi, a prank, and some kind of narcotics. Suffice to say, she got the wrong the prescription.

Yuka:Exactly what happened at the time went like this her, Tewi, a prank, and some kind of narcotics. Suffice to say, I got the wrong prescription.

Kaguya is full of these.

From the recent in the Gensokyo 20XX Series, we have several, usually involving the kids or something occurring with some of the other characters.

From the sidestories and one-shots, we usually get these with pranks but the one that is particularly eye-catching is from the one-shot Our Fault, where the results of a noodle incident sets up the whole plot and, whilst Chen is narrating, she never reveals what it was.

From the same one-shot, Yukari had a bad experience with water once, leading for her wear a football helmet and floaties in whenever she gets in the water. This has the effect of making her swim slower.

In Like A Girl, Numair experiments with a new spell in order to change Joren back into a boy. He reappears several minutes later in a cloud of smoke, covered with soot, remarking that he is never doing anything like that again.

In the Legion of Lawndale Heroes Daria fic, the characters occasionally make mention of "the Porterhouse Challenge", which may be related to the VERY large steak that a number of restaurants offer (hilariously large, in some cases) with the caveat that if a person can eat it in one sitting, the meal is free.

There was the time with classroom blowing up in middle school and "the Xiao-Long fire" and apparently Satsuki had something to do with those incidents, according to Ryuuko.

There was the last time Mako held fireworks for Ryuuko

Ryuuko: Nah, she can’t hold fireworks… Not after last time.

Various other things

Ragyou: No mindless destruction of public or private property, no pretending to be public officials. No calling Nicaragua, no adopting rabid animals. And absolutely no sneaking into the morgue and touching cadavers! No digging up corpses of any dead things to prove to Nui that they’re actually dead! Grandpa and Professor Ratigan are dead!

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